Thermomechanical pulping (TMP) and chemi-thermomechanical pulping (CTMP) processes refine fibrous material at high consistency (HC), typically having 20 percent (20%) or more fiber by weight of the pulp suspension passing through the mainline and rejects refiners. With HC refining, the pulp suspension is a fibrous mass and is transported by a pressurized blowline or screw conveyor which can handle such masses. In contrast, pulp suspensions in low consistency (LC) refining flow as a liquid slurry that can be moved by pumps.
Mechanically refining pulp at a high consistency requires a large amount of energy that is expended primarily in frictional heat losses associated with viscoelastic deformations of the pulp in the refining zone. These frictional heat losses result in a large amount of energy that is not applied directly to refining pulp. Typically less than 10% to 15% of the electric energy applied in a HC TMP or CTMP refiner is directly applied to refining the pulp.
One way to reduce the energy demand of conventional TMP and CTMP processes is to install LC refining following primary and secondary HC refining and the latency removal chest. Using this approach, energy savings of 5% to 8% or even more have been reported without sacrificing pulp properties. Similar tear strength and sometimes even slightly higher tensile strength and lower shive levels have been reported. This third stage low consistency refining approach has found relatively wide acceptance particularly in North America and usually provides increased capacity for TMP or CTMP lines. There is a need for improved energy efficiency in refiner-based pulping.
Alkaline peroxide is conventionally used for brightening of mechanical pulps after refining. Moldenius [2] and U.S. Pat. No. 4,734,160 found that the brightening process of mechanical pulps with hyper alkaline peroxide (pH 12-13) can serve to simultaneously enhance both brightness and tensile strength, depending on the alkali and peroxide charges used. The application of alkaline peroxide has also been used with TMP screen rejects [4].
US 2009/0032207 describes a method for producing mechanical or chemi-mechanical pulp as raw material for paper or cardboard in which the pulp is fibrillated and the fibrillated pulp is bleached in alkaline conditions. The pulp is screened to separate the rejects from the accepts. The rejects are bleached separate from the accepts, and, after that, the bleached rejects are remixed with the accepts.
U.S. Pat. No. 6,743,332 describes a process for producing mechanical pulp using a bleaching liquor comprising a hydrogen peroxide-magnesium hydroxide or soda ash mixture (replacing sodium hydroxide), at a temperature of 85-160 degrees C., and up to a pH of about 9-10.5. The pH range was selected to prevent peroxide decomposition and alkali darkening.
US 2008/0035286 discloses a method for processing lignocellulosic material involving the use of a non-compression vessel or digester to chemically precondition wood chips with stabilizers, washing, fiberizing the preconditioned chips and processing the fiberized material using a high consistency refiner in the presence of alkali peroxide, followed by low consistency refining.